This invention relates to a process for preparing a solid olefin polymerization catalyst component having improved activity and selectivity.
There are numerous prior art proposals to provide olefin polymerization catalysts by combining a solid component comprising at least magnesium, titanium and halogen, particularly chlorine with an activating organoaluminum compound. These may be referred to as supported coordination catalysts or catalyst systems. The activity and stereospecific performance of such compositions is generally improved by incorporating an electron donor (Lewis base) in the solid component and by employing as a third catalyst component an electron donor which may be complexed in whole or in part with the activating organoaluminum compound.
For convenience of reference, the solid titanium-containing constituent of such catalysts is referred to herein as "procatalyst", the organoaluminum compound, whether used separately or partially or totally complexed with an electron donor, as "cocatalyst", and the electron donor compound, whether used separately or partially or totally complexed with the organoaluminum compound, as "selectivity control agent" (SCA).
Supported coordination catalysts of this type are disclosed in numerous patents. The catalyst systems of this type which have been disclosed in the prior art generally are able to produce olefin polymers in high yield and, in the case of catalysts for polymerization of propylene or higher alpha-olefins, with high selectivity to stereoregular polymer. However, further improvements in productivity at high stero-regularity are still being sought.
The objective of workers in this art is to provide catalyst systems which exhibit sufficiently high activity to permit the production of olefin polymers in such high yield as to obviate the necessity of extracting residual catalyst components in the deashing step. In the case of propylene and higher olefins, an equally important objective is to provide catalyst systems of sufficiently high selectivity toward isotactic or otherwise stereoregular products to obviate the necessity of extracting atactic polymer components.
Although many chemical combinations provide active catalyst systems, practical considerations have led workers in the art to concentrate on certain preferred components. The procatalysts typically comprise magnesium chloride, titanium chloride, generally in tetravalent form, and as electron donor an aromatic ester such as ethyl benzoate or ethyl-p-toluate. The cocatalyst typically is an aluminum trialkyl such as aluminum triethyl or aluminum tri-isobutyl, often used at least partially complexed with selectivity control agent. The selectivity control agent typically is an aromatic ester such as ethyl-paramethoxy-benzoate(ethyl anisate) or methyl-p-toluate.
While the selection of a cocatalyst and selectivity control agent affects the performance of those catalyst systems, the component which appears to be subject to most significant improvement with respect to activity and productivity of the system is the procatalyst.
Preferred methods of preparing such procatalyst are described in U.S. Pat. Nos. 4,329,253, 4,393,182, 4,400,302, and 4,414,132. These procatalysts are highly active and stereospecific. The typical manner of preparing such procatalysts involves the reaction of the magnesium compound, titanium tetrachloride and electron donor in the presence of a halohydrocarbon. The resulting solid particle are then contacted with additional quantities of TiCl.sub.4 and the preparations are completed by washing off excess TiCl.sub.4 using light hydrocarbons (e.g., isooctane and isopentane) and drying.
The procatalysts prepared in this way have excellent polymerization activity (polymer yield) and stereospecific performance (isotactic content). However, for some applications the activity and selectivity still need improvement.